1. Field of the Invention
The present invention relates to a superior scanning probe microscope that changes oscillations of a cantilever having a microscopic probe at a distal end using physical characteristics acting between the probe and a sample surface when the probe approaches the sample while oscillating the cantilever so as to measure shape and physical properties of the surface of the sample.
2. Description of Related Art
Dynamic measurements (Dynamic Force Microscopy: DFM mode) in Scanning Probe Microscopy (SPM) where displacement and other physical quantities are detected while driving a cantilever fitted with a probe at a distal end in the vicinity of it's resonant frequency so as to obtain an image and carry out observation is well-known. With this type of scanning probe microscope in the related art, a cantilever having a microscopic probe at a distal end is fixed to a cantilever holder. Then cantilever is then made to oscillate at a frequency in the vicinity of its resonant frequency by oscillating means employing a piezoelectric element, etc., and the amplitude at this time is measured by displacement detection means. Optical lever methods where the back of the cantilever is a mirrored surface are used as the displacement detection means. A sample is mounted on a stage having a triaxial fine adjustment mechanism constructed from a piezoelectric element, etc., that performs X-Y plane scanning and Z-position adjustment. When the sample comes close to the probe as a result of a coarse adjustment mechanism, when the probe and the sample are sufficiently close, physical forces, such as interatomic forces, act between the sample and the probe. The amplitude of oscillation of the oscillated cantilever therefore changes due to the cantilever being subjected to such physical forces. The force acting at this time depends on the distance between the probe and the sample. When the probe and the sample come within a region where an interatomic force acts, when distance between the sample and the probe is controlled using a Z-position adjustment function so that amplitude of oscillation of the cantilever is normally fixed while scanning within a two-dimensional plane using a fine adjustment mechanism, the extent of this control (displacement) corresponds to the unevenness of the sample surface. A TOPO (uneven shape) image of the sample surface can then be obtained by putting the amount of control during this time into the form of an image.
In the case of performing measurement dynamic (DFM mode) using as scanning probe microscope, a frequency characteristic for the vicinity of the resonance point of the cantilever or probe is obtained (as shown in FIG. 12) prior to the measurement. A resonance frequency ω0and a quality factor (“Q”) value are obtained for the cantilever and the probe from the waveform for this frequency characteristic. The Q-value can normally be obtained from the following formula;Q=ω0/ (ω2−ω1)
where ω0 is resonance frequency of the cantilever, ω1 and ω2 are frequencies at points of intersection of portions where A/21/2 (note: 21/2expresses the square root of 2) when amplitude of the resonance frequency is taken to be A and a frequency characteristic curve. It can be understood from this equation that Q-value is decided by the width of the resonance point peak and that Q-value is larger for a steeper peak. When the cantilever and probe approach the sample while being oscillated at a frequency in the vicinity of the resonance point, as shown in FIG. 12, the resonance frequency is shifted and the amplitude changes due to interatomic forces between the sample and the probe. When the force acting between the sample and the probe during this time is a force of attraction, the resonance frequency falls, and on the contrary, becomes high in the case of forces of repulsion. With the cantilever and probe, as shown in FIG. 13, the gradient of the waveform is steeper for a larger Q-value. On the other hand, when considering the overall system including the control system, if the Q value is too high, response becomes unstable, the system cannot achieve follow ability and this becomes the cause of oscillation. It is therefore necessary to optimize the Q-value in order to achieve both good response and sensitivity in a dynamic scanning probe microscope system.
However, the Q-value (Q-factor) that is a resonance characteristic typically depends on the mechanical structure of the cantilever, excitation transmission efficiency, and operating environment, etc. In, for example, high-resolution and high sensitivity measurements (Magnetic Force Microscopy: MFM etc. of phase measurements where sensitivity depends on Q-value), measurement takes place with the Q-value raised. However, restrictions are imposed by the mechanical structure of the cantilever and the excitation transmission efficiency that prevent this. Conversely, in dynamic (DFM mode) SPM driving in a vacuum environment, the Q-value can be made in the order of one to two digits higher compared to the atmosphere environment because the influence of the air resistance is dramatically reduced. In this case, this is effective with regards to sensitivity but measurement response falls from the viewpoint of SPM control (the speed of the feedback for tracing the shape). The probe and the sample then collide so as to cause resolution to fall or cause excitation rendering measurement impossible. In order to prevent this it is necessary to dramatically lower the scanning speed, which is problematic in reality and presents many points of difficulty.
With SPM it is possible not just to measure surface shape but also to measure various physical properties synchronously or asynchronously. However, in the related art, as described above, control to perform synchronous measurement (for example, MFM) of a surface shape by lowering Q-value or perform measurements by increasing the Q-value could not be achieved. It was therefore not possible to set optimum conditions for mutually incompatible conditions.
This applicant has therefore previously applied for Japanese Patent Application JP10-148117 (Japanese Patent Laid-open Publication JP11-337560A) entitled “scanning probe microscope” under these conditions. This invention provides a configuration that enables control of a Q-value of a cantilever for a cantilever holder of a scanning probe microscope where the amplitude of oscillation of a cantilever having a microscopic probe at a distal end is made to change using physical characteristics acting between the probe and a sample surface when the probe approaches the sample while oscillating the cantilever so as to measure shape and physical properties of the surface of the sample. To achieve this, a first actuator is fixed to a substrate, and a cantilever is mounted in such a manner that excitation force of the first actuator is transmitted to the cantilever. A flat part of the cantilever then makes contact with a resilient member. A second actuator is then provided at another flat part so as to push the cantilever against the resilient member. This made Q-control possible. However, execution of optimal value control is not straight forward because it is necessary to adjust the mechanical means for pressing the cantilever against the resilient member using the second actuator.
In the paper “Enhanced imaging of DNA via active quality factor control” published by A. D. L. Humphris etc. in the technical journal “Surface Science VOL. 491, No.3, pp. 468-472(2001)”, as shown in FIG. 14, a signal is made by passing a displacement detection signal of a cantilever of a dynamically driven SPM through a variable phase-shifter and a variable gain amplifier. This signal is then superimposed with a forced oscillation signal of a conventional oscillator and is applied as an excitation signal. Viscosity is then controlled by a feedback loop passing via the variable phase-shifter and the variable gain amplifier so as to control Q-value. In this non-patent document, an example is introduced of straightforward measurement of DNA etc. where a Q-value that is too low in a fluid is raised.
On homepage of Veeco Instruments (http://www.veeco.com/html/datasheet_nanoscopeIV.asp), an example is introduced of increasing MFM sensitivity through magnetic domain observation of magnetic tapes in the atmosphere through Q-control using a NanoScope IV as an SPM controller.
However, this technology does not describe anything more than simply increasing amplitude of a mechanism resonance point of a cantilever by controlling the Q-value. In this technology, a Q-value (Q-factor) indicating amplitude quality at the mechanical resonance point is prepared (fixed at a desired value) prior to having the probe make an approach to become extremely close to the sample surface. The cantilever (probe) is then brought close to the surface of the sample in the same way as for a conventional SPM. Mutual interaction (attraction, repulsion) with the sample as the probe comes near is then detected, contact is made at a desired extremely weak force, and an image acquisition preparation state is entered. However, the Q-value (Q0) occurring at the distance (referred to as Z0) between the sample surface and the probe when setting the first Q-value falls as the probe approaches the sample. The Q-value then changes as far as QE when the approach to the sample is complete (the height is ZE). When the extent of the change in the Q-value during this time is taken to be ΔQ, then ΔQ is given by the function change in distance (ΔZ=Z0−Z). Typically, ΔZ also tends to be large when ΔQ is large. The reason for this is that a substantially ideal oscillation amplitude can be obtained at Z0 where the air resistance is small but air resistance with respect to the oscillation becomes larger as the distance between the sample and the probe becomes smaller, the oscillating action of the cantilever is hindered, and the Q-value inevitably falls. Therefore, even though the intention is to set the desired Q value=Q0, when an image is actually being acquired, the Q-value changes to Q-value=QE, meaning that it is only possible to obtain the resulting Q0/QE for the anticipated Q-value. Namely, even if the Q-value is diligently set high, after an approach this becomes a low Q-value and sensitivity is not improved to the extent anticipated. Further, magnetic force gradient also fluctuates because of variation in the effective Q-value due to carrying out Q-control which presents problems with regards to data reproducibility. This problem is more marked under Q-control MFM under an atmosphere environment, with the extent of these differences also presenting problems broadly speaking to DFM measurements on the whole.
In Q-control MFM of the related art, it is difficult to set a stable desired Q-value when executing measurements. Therefore, even when a high Q-value is set, response is sacrificed and in particular, stability of shape measurement deteriorates. Further, when a low Q-value is set, there is a problem that high sensitivity in measurement of magnetic force gradient is sacrificed.